1. Field of the Invention
This invention relates generally to heliostats that are useful for concentrating reflected solar energy upon a remote absorber, and more particularly a rim supported heliostat that may be turned to a face-down stow position.
2. Description of Prior Art
Conventional solar thermal power systems employ arrays of individually steered reflectors or heliostats and a tower-mounted central receiver. In a megawatt sized system of commercial import a field may include hundreds or even thousands of heliostats. Such an array comprises a heliostat field which redirects radiant solar energy to the receiver. The individual heliostat in such a field has typically comprised silver backed glass mirrors supported on a steel frame which is connected to a two-axis tracking drive system and mounted to a pedestal anchored in reinforced concrete. These glass/metal heliostat assemblies are expensive. They are also relatively massive and involve structures such as beams and trusses which represent a significant amount of material in bending--which is not very efficient structurally. Since the heliostat field represents a significant part of the system investment, a lighter, less expensive, and more efficient heliostat assembly would bring a significant improvement in cost effectiveness and will help solar thermal systems become truly viable.
The recent development of stretched membrane heliostats, in which a sheet material such as thin gauge metal is supported in tension upon a rigid frame, has provided heliostats that are light-weight and less expensive. However, the benefits of the stretched membrane concept cannot be beneficially incorporated into a system until a complementary, efficient and effective assembly for mounting and driving this type of heliostat is provided.
Another concern in conventional central receiver systems is the need to keep the heliostat reflective surfaces clean and to protect them from the effects of adverse weather conditions such as hail, rain and windstorms. Where expensive reflective coatings are used the concern is even greater. Again, given the large numbers of heliostats in a field, this is a significant concern.